Aluminum-copper-lithium alloys

ABSTRACT

Improved aluminum-copper-lithium alloys are disclosed. The alloys may include 3.4-4.2 wt. % Cu, 0.9-1.4 wt. % Li, 0.3-0.7 wt. % Ag, 0.1-0.6 wt. % Mg, 0.2-0.8 wt. % Zn, 0.1-0.6 wt. % Mn, and 0.01-0.6 wt. % of at least one grain structure control element, the balance being aluminum and incidental elements and impurities. The alloys achieve an improved combination of properties over prior art alloys.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 13/368,586, filed Feb. 8, 2012, which is a continuation of U.S.patent application Ser. No. 12/328,622, filed Dec. 4, 2008, now U.S.Pat. No. 8,118,950, entitled “IMPROVED ALUMINUM-COPPER-LITHIUM ALLOYS”,which claims priority to U.S. Provisional Patent Application No.60/992,330, filed Dec. 4, 2007, and entitled “IMPROVED ALUMINUM ALLOYS”,and is related to PCT Patent Application No. PCT/US08/85547, filed Dec.4, 2008. Each of the above-identified patent applications isincorporated herein by reference in its entirety.

BACKGROUND

Aluminum alloys are useful in a variety of applications. However,improving one property of an aluminum alloy without degrading anotherproperty often proves elusive. For example, it is difficult to increasethe strength of an alloy without decreasing the toughness of an alloy.Other properties of interest for aluminum alloys include corrosionresistance, density and fatigue, to name a few.

SUMMARY OF THE DISCLOSURE

Broadly, the present disclosure relates to aluminum-copper-lithiumalloys having an improved combination of properties.

In one aspect, the aluminum alloy is a wrought aluminum alloy consistingessentially of 3.4-4.2 wt. % Cu, 0.9-1.4 wt. % Li, 0.3-0.7 wt. % Ag,0.1-0.6 wt. % Mg, 0.2-0.8 wt. % Zn, 0.1-0.6 wt. % Mn, and 0.01-0.6 wt. %of at least one grain structure control element, the balance beingaluminum and incidental elements and impurities. The wrought product maybe an extrusion, plate, sheet or forging product. In one embodiment, thewrought product is an extruded product. In one embodiment, the wroughtproduct is a plate product. In one embodiment, the wrought product is asheet product. In one embodiment, the wrought product is a forging.

In one approach, the alloy is an extruded aluminum alloy. In oneembodiment, the alloy has an accumulated cold work of not greater thanan equivalent of 4% stretch. In other embodiments, the alloy has anaccumulated cold work of not greater than an equivalent of 3.5% or notgreater than an equivalent of 3% or even not greater than an equivalentof 2.5% stretch. As used herein, accumulated cold work means cold workaccumulated in the product after solution heat treatment.

In some embodiments, the aluminum alloy includes at least about 3.6 or3.7 wt. %, or even at least about 3.8 wt. % Cu. In some embodiments, thealuminum alloy includes not greater than about 4.1 or 4.0 wt. % Cu. Insome embodiments, the aluminum alloy includes copper in the range offrom about 3.6 or 3.7 wt. % to about 4.0 or 4.1 wt. %. In oneembodiment, the aluminum alloy includes copper in the range of fromabout 3.8 wt. % to about 4.0 wt. %.

In some embodiments, the aluminum alloy includes at least about 1.0 or1.1 wt. % Li. In some embodiments, the aluminum alloy includes notgreater than about 1.3 or 1.2 wt. % Li. In some embodiments, thealuminum alloy includes lithium in the range of from about 1.0 or 1.1wt. % to about 1.2 or 1.3 wt. %.

In some embodiments, the aluminum alloy includes at least about 0.3 or0.35 or 0.4 or 0.45 wt. % Zn. In some embodiments, the aluminum alloyincludes not greater than about 0.7 or 0.65 or 0.6 or 0.55 wt. % Zn. Insome embodiments, the aluminum alloy includes zinc in the range of fromabout 0.3 or 0.4 wt. % to about 0.6 or 0.7 wt. %.

In some embodiments, the aluminum alloy includes at least about 0.35 or0.4 or 0.45 wt. % Ag. In some embodiments, the aluminum alloy includesnot greater than about 0.65 or 0.6 or 0.55 wt. % Ag. In someembodiments, the aluminum alloy includes silver in the range of fromabout 0.35 or 0.4 or 0.45 wt. % to about 0.55 or 0.6 or 0.65 wt. %.

In some embodiments, the aluminum alloy includes at least about 0.2 or0.25 wt. % Mg. In some embodiments, the aluminum alloy includes notgreater than about 0.5 or 0.45 wt. % Mg. In some embodiments, thealuminum alloy includes magnesium in the range of from about 0.2 or 0.25wt. % to about 0.45 or 0.5 wt. %.

In some embodiments, the aluminum alloy includes at least about 0.15 or0.2 wt. % Mn. In some embodiments, the aluminum alloy includes notgreater than about 0.5 or 0.4 wt. % Mn. In some embodiments, thealuminum alloy includes manganese in the range of from about 0.15 or 0.2wt. % to about 0.4 or 0.5 wt. %.

In one embodiment, the grain structure control element is Zr. In some ofthese embodiments, the aluminum alloy includes 0.05-0.15 wt. % Zr.

In one embodiment, the impurities include Fe and Si. In some of theseembodiments, the alloy includes not greater than about 0.06 wt. % Si(e.g., ≦0.03 wt. % Si) and not greater than about 0.08 wt. % Fe (e.g.,≦0.04 wt. % Fe).

The aluminum alloy may realize an improved combination of mechanicalproperties and corrosion resistant properties. In one embodiment, analuminum alloy realizes a longitudinal tensile yield strength of atleast about 86 ksi. In one embodiment, the aluminum alloy realizes anL-T plane strain fracture toughness of at least about 20 ksi√in. In oneembodiment, the aluminum alloy realizes a typical tension modulus of atleast about 11.3×10³ ksi and a typical compression modulus of at leastabout 11.6×10³ ksi. In one embodiment, the aluminum alloy has a densityof not greater than about 0.097 lbs./in³. In one embodiment, thealuminum alloy has a specific strength of at least about 8.66×10⁵ in. Inone embodiment, the aluminum alloy realizes a compressive yield strengthof at least about 90 ksi. In one embodiment, the aluminum alloy isresistant to stress corrosion cracking. In one embodiment, the aluminumalloy achieves a MASTMAASIS rating of at least EA. In one embodiment,the alloy is resistant to galvanic corrosion. In some aspects, a singlealuminum alloy may realize numerous ones (or even all) of the aboveproperties. In one embodiment, the aluminum alloy at least realizes alongitudinal strength of at least about 84 ksi, an L-T plane strainfracture toughness of at least about 20 ksi√in, is resistant to stresscorrosion cracking and is resistant to galvanic corrosion.

These and other aspects, advantages, and novel features of the newalloys are set forth in part in the description that follows, and becomeapparent to those skilled in the art upon examination of the followingdescription and figures, or may be learned by production of or use ofthe alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic view illustrating one embodiment of a testspecimen for use in fracture toughness testing.

FIG. 1 b is a dimension and tolerance table relating to FIG. 1 a.

FIG. 2 is a graph illustrating typical tensile yield strength versustensile modulus values for various alloys.

FIG. 3 is a graph illustrating typical specific tensile yield strengthvalues for various alloys.

FIG. 4 is a schematic view illustrating one embodiment of a test couponfor use in notched S/N fatigue testing.

FIG. 5 is a graph illustrating the galvanic corrosion resistance ofvarious alloys.

DETAILED DESCRIPTION

Reference will now be made in detail to the accompanying drawings, whichat least assist in illustrating various pertinent embodiments of the newalloy.

Broadly, the instant disclosure relates to aluminum-copper-lithiumalloys having an improved combination of properties. The aluminum alloysgenerally comprise (and in some instances consist essentially of)copper, lithium, zinc, silver, magnesium, and manganese, the balancebeing aluminum, optional grain structure control elements, optionalincidental elements and impurities. The composition limits of severalalloys useful in accordance with the present teachings are disclosed inTable 1, below. The composition limits of several prior art alloys aredisclosed in Table 2, below. All values given are in weight percent.

TABLE 1 New Alloy Compositions Alloy Cu Li Zn Ag Mg Mn A 3.4-4.2%0.9-1.4% 0.2-0.8% 0.3-0.7% 0.1-0.6% 0.1-0.6% B 3.6-4.1% 1.0-1.3%0.3-0.7% 0.4-0.6% 0.2-0.5% 0.1-0.4% C 3.8-4.0% 1.1-1.2% 0.4-0.6%0.4-0.6% 0.25-0.45% 0.2-0.4%

TABLE 2 Prior Art Extruded Alloy Compositions Alloy Cu Li Zn Ag Mg Mn2099 2.4-3.0% 1.6-2.0% 0.4-1.0% —  0.1-0.5% 0.1-0.5% 2195 3.7-4.3%0.8-1.2% Max 0.25 0.25-0.6% 0.25-0.8% Max 0.25 wt. % as wt. % asimpurity impurity 2196 2.5-3.3% 1.4-2.1% Max 0.35 0.25-0.6% 0.25-0.8%Max 0.35 wt. % as wt. % as impurity impurity 7055 2.0-2.6% — 7.6-8.4% — 1.8-2.3% Max 0.05 wt. % as impurity 7150 1.9-2.5% — 5.9-6.9% — 2.0-2.7% Max 0.10 wt. % as impurity

The alloys of the present disclosure generally include the statedalloying ingredients, the balance being aluminum, optional grainstructure control elements, optional incidental elements and impurities.As used herein, “grain structure control element” means elements orcompounds that are deliberate alloying additions with the goal offorming second phase particles, usually in the solid state, to controlsolid state grain structure changes during thermal processes, such asrecovery and recrystallization. Examples of grain structure controlelements include Zr, Sc, V, Cr, and Hf, to name a few.

The amount of grain structure control material utilized in an alloy isgenerally dependent on the type of material utilized for grain structurecontrol and the alloy production process. When zirconium (Zr) isincluded in the alloy, it may be included in an amount up to about 0.4wt. %, or up to about 0.3 wt. %, or up to about 0.2 wt. %. In someembodiments, Zr is included in the alloy in an amount of 0.05-0.15 wt.%. Scandium (Sc), vanadium (V), chromium (Cr), and/or hafnium (Hf) maybe included in the alloy as a substitute (in whole or in part) for Zr,and thus may be included in the alloy in the same or similar amounts asZr.

While not considered a grain structure control element for the purposesof this application, manganese (Mn) may be included in the alloy inaddition to or as a substitute (in whole or in part) for Zr. When Mn isinclude in the alloy, it may be included in the amounts disclosed above.

As used herein, “incidental elements” means those elements or materialsthat may optionally be added to the alloy to assist in the production ofthe alloy. Examples of incidental elements include casting aids, such asgrain refiners and deoxidizers.

Grain refiners are inoculants or nuclei to seed new grains duringsolidification of the alloy. An example of a grain refiner is a ⅜ inchrod comprising 96% aluminum, 3% titanium (Ti) and 1% boron (B), wherevirtually all boron is present as finely dispersed TiB₂ particles.During casting, the grain refining rod is fed in-line into the moltenalloy flowing into the casting pit at a controlled rate. The amount ofgrain refiner included in the alloy is generally dependent on the typeof material utilized for grain refining and the alloy productionprocess. Examples of grain refiners include Ti combined with B (e.g.,TiB₂) or carbon (TiC), although other grain refiners, such as Al—Timaster alloys may be utilized. Generally, grain refiners are added in anamount of ranging from 0.0003 wt. % to 0.005 wt. % to the alloy,depending on the desired as-cast grain size. In addition, Ti may beseparately added to the alloy in an amount up to 0.03 wt. % to increasethe effectiveness of grain refiner. When Ti is included in the alloy, itis generally present in an amount of up to about 0.10 or 0.20 wt. %.

Some alloying elements, generally referred to herein as deoxidizers, maybe added to the alloy during casting to reduce or restrict (and is someinstances eliminate) cracking of the ingot resulting from, for example,oxide fold, pit and oxide patches. Examples of deoxidizers include Ca,Sr, and Be. When calcium (Ca) is included in the alloy, it is generallypresent in an amount of up to about 0.05 wt. %, or up to about 0.03 wt.%. In some embodiments, Ca is included in the alloy in an amount of0.001-0.03 wt % or 0.05 wt. %, such as 0.001-0.008 wt. % (or 10 to 80ppm). Strontium (Sr) may be included in the alloy as a substitute for Ca(in whole or in part), and thus may be included in the alloy in the sameor similar amounts as Ca. Traditionally, beryllium (Be) additions havehelped to reduce the tendency of ingot cracking, though forenvironmental, health and safety reasons, some embodiments of the alloyare substantially Be-free. When Be is included in the alloy, it isgenerally present in an amount of up to about 20 ppm.

Incidental elements may be present in minor amounts, or may be presentin significant amounts, and may add desirable or other characteristicson their own without departing from the alloy described herein, so longas the alloy retains the desirable characteristics described herein. Itis to be understood, however, that the scope of this disclosure shouldnot/cannot be avoided through the mere addition of an element orelements in quantities that would not otherwise impact on thecombinations of properties desired and attained herein.

As used herein, impurities are those materials that may be present inthe alloy in minor amounts due to, for example, the inherent propertiesof aluminum or and/or leaching from contact with manufacturingequipment. Iron (Fe) and silicon (Si) are examples of impuritiesgenerally present in aluminum alloys. The Fe content of the alloy shouldgenerally not exceed about 0.25 wt. %. In some embodiments, the Fecontent of the alloy is not greater than about 0.15 wt. %, or notgreater than about 0.10 wt. %, or not greater than about 0.08 wt. %, ornot greater than about 0.05 or 0.04 wt. %. Likewise, the Si content ofthe alloy should generally not exceed about 0.25 wt. %, and is generallyless than the Fe content. In some embodiments, the Si content of thealloy is not greater than about 0.12 wt. %, or not greater than about0.10 wt. %, or not greater than about 0.06 wt. %, or not greater thanabout 0.03 or 0.02 wt. %.

Except where stated otherwise, the expression “up to” when referring tothe amount of an element means that that elemental composition isoptional and includes a zero amount of that particular compositionalcomponent. Unless stated otherwise, all compositional percentages are inweight percent (wt. %).

The alloys can be prepared by more or less conventional practicesincluding melting and direct chill (DC) casting into ingot form.Conventional grain refiners, such as those containing titanium andboron, or titanium and carbon, may also be used as is well-known in theart. After conventional scalping, lathing or peeling (if needed) andhomogenization, these ingots are further processed into wrought productby, for example, hot rolling into sheet (≦0.249 inch) or plate (≧0.250inch) or extruding or forging into special shaped sections. In the caseof extrusions, the product may be solution heat treated (SHT) andquenched, and then mechanically stress relieved, such as by stretchingand/or compression up to about 4% permanent strain, for example, fromabout 1 to 3%, or 1 to 4%. Similar SHT, quench, stress relief andartificial aging operations may also be completed to manufacture rolledproducts (e.g., sheet/plate) and/or forged products.

The new alloys disclosed herein achieve an improved combination ofproperties relative to 7xxx and other 2xxx series alloys. For example,the new alloys may achieve an improved combination of two or more of thefollowing properties: ultimate tensile strength (UTS), tensile yieldstrength (TYS), compressive yield strength (CYS), elongation (El)fracture toughness (FT), specific strength, modulus (tensile and/orcompressive), specific modulus, corrosion resistance, and fatigue, toname a few. In some instances, it is possible to achieve at least someof these properties without high amounts of accumulated cold work, suchas those used for prior Al—Li products such as 2090-T86 extrusions.Realizing these properties with low amounts of accumulated cold work isbeneficial in extruded products. Extruded products generally cannot becompressively worked, and high amounts of stretch make it highlydifficult to maintain dimensional tolerances, such as cross-sectionalmeasurements and attribute tolerances, including angularity andstraightness, as described in the ANSI H35.2 specification.

With respect to strength and elongation, the alloys may achieve alongitudinal (L) ultimate tensile strength of at least about 92 ksi, oreven at least about 100 ksi. The alloys may achieve a longitudinaltensile yield strength of at least about 84 ksi, or at least about 86ksi, or at least about 88 ksi, or at least about 90 ksi, or even atleast about 97 ksi. The alloys may achieve a longitudinal compressiveyield strength of at least about 88 ksi, or at least about 90 ksi, or atleast about 94 ksi, or even at least about 98 ksi. The alloys mayachieve an elongation of at least about 7%, or even at least about 10%.In one embodiment, the ultimate tensile strength and/or tensile yieldstrength and/or elongation is measured in accordance with ASTM E8 and/orB557, and at the quarter-plane of the product. In one embodiment, theproduct (e.g., the extrusion) has a thickness in the range of0.500-2.000 inches. In one embodiment, the compressive yield strength ismeasured in accordance with ASTM E9 and/or E111, and at thequarter-plane of the product. It may be appreciated that strength canvary somewhat with thickness. For example, thin (e.g., <0.500 inch) orthick products (e.g., >3.0 inches) may have somewhat lower strengthsthan those described above. Nonetheless, those thin or thick productsstill provide distinct advantages relative to previously available alloyproducts.

With respect to fracture toughness, the alloys may achieve along-transverse (L-T) plane strain fracture toughness of at least about20 ksi√in., or at least about 23 ksi√in., or at least about 27 ksi√in.,or even at least about 31 ksi√in. In one embodiment, the fracturetoughness is measured in accordance with ASTM E399 at the quarter-plane,and with the specimen configuration illustrated in FIG. 1 a. It may beappreciated that fracture toughness can vary somewhat with thickness andtesting conditions. For example, thick products (e.g., >3.0 inches) mayhave somewhat lower fracture toughness than those described above.Nonetheless, those thick products still provide distinct advantagesrelative to previously available products.

With respect to FIG. 1 a, a dimension and tolerances table is providedin FIG. 1 b. Note 1 of FIG. 1 a states grains in this direction for L-Tand L-S specimens. Note 2 of FIG. 1 a states grain in this direction forT-L and T-S specimens. Note 3 of FIG. 1 a states S notch dimension shownis maximum, if necessary may be narrower. Note 4 of FIG. 1 a states tocheck for residual stress, measure and record height (2H) of specimen atposition noted both before and after machining notch. All tolerances areas follows (unless otherwise noted): 0.0=+/−0.1; 0.00=+/−0.01;0.000=+/−0.005.

With respect to specific tensile strength, the alloys may realize adensity of not greater than about 0.097 lb/in³, such as in the range of0.096 to 0.097 lb/in³. Thus, the alloys may realize a specific tensileyield strength of at least about 8.66×10⁵ in. ((84 ksi*1000=84,000lb./in)/(0.097 lb./in³=about 866,000 in.), or at least about 8.87×10⁵in., or at least about 9.07∴10⁵ in., or at least about 9.28×10⁵ in., oreven at least about 10.0×10⁵ in.

With respect to modulus, the alloys may achieve a typical tensilemodulus of at least about 11.3 or 11.4×10³ ksi. The alloys may realize atypical compressive modulus of at least about 11.6 or 11.7×10³ ksi. Inone embodiment, the modulus (tensile or compressive) may be measured inaccordance with ASTM E111 and/or B557, and at the quarter-plane of thespecimen. The alloys may realize a specific tensile modulus of at leastabout 1.16×10⁸ in. ((11.3×10³ ksi*1000=11.3*10⁶ lb./in.)/(0.097lb./in³=about 1.16×10⁸ in.). The alloys may realize a specificcompression modulus of at least about 1.19×10⁸ in.

With respect to corrosion resistance, the alloys may be resistant tostress corrosion cracking. As used herein, resistant to stress corrosioncracking means that the alloys pass an alternate immersion corrosiontest (3.5 wt. % NaCl) while being stressed (i) at least about 55 ksi inthe LT direction, and/or (ii) at least about 25 ksi in the ST direction.In one embodiment, the stress corrosion cracking tests are conducted inaccordance with ASTM G47.

With respect to exfoliation corrosion resistance, the alloys may achieveat least an “EA” rating, or at least an “N” rating, or even at least an“P” rating in a MASTMAASIS testing process for either or both of the T/2or T/10 planes of the product, or other relevant test planes andlocations. In one embodiment, the MASTMAASIS tests are conducted inaccordance with ASTM G85-Annex 2 and/or ASTM G34.

The alloys may realize improved galvanic corrosion resistance, achievinglow corrosion rates when connected to a cathode, which is known toaccelerate corrosion of aluminum alloys. Galvanic corrosion refers tothe process in which corrosion of a given material, usually a metal, isaccelerated by connection to another electrically conductive material.The morphology of this type of accelerated corrosion can vary dependingon the material and environment, but could include pitting,intergranular, exfoliation, and other known forms of corrosion. Oftenthis acceleration is dramatic, causing materials that would otherwise behighly resistant to corrosion to deteriorate rapidly, thereby shorteningstructure lifetime. Galvanic corrosion resistance is a consideration formodern aircraft designs. Some modern aircraft may combine many differentmaterials, such as aluminum with carbon fiber reinforced plasticcomposites (CFRP) and/or titanium parts. Some of these parts are verycathodic to aluminum, meaning that the part or structure produced froman aluminum alloy may experience accelerated corrosion rates when inelectrical communication (e.g., direct contact) with these materials.

In one embodiment, the new alloy disclosed herein is resistant togalvanic corrosion. As used herein, “resistant to galvanic corrosion”means that the new alloy achieves at least 50% lower current density(uA/cm²) in a quiescent 3.5% NaCl solution at a potential of from about−0.7 to about −0.6 (volts versus a saturated calomel electrode (SCE))than a 7xxx alloy of similar size and shape, and which 7xxx alloy has asimilar strength and toughness to that of the new alloy. Some 7xxxalloys suitable for this comparative purpose include 7055 and 7150. Thegalvanic corrosion resistance tests are performed by immersing the alloysample in the quiescent solution and then measuring corrosion rates bymonitoring electrical current density at the noted electrochemicalpotentials (measured in volts vs. a saturated calomel electrode). Thistest simulates connection with a cathodic material, such as thosedescribed above. In some embodiments, the new alloy achieves at least75%, or at least 90%, or at least 95%, or even at least 98% or 99% lowercurrent density (uA/cm²) in a quiescent 3.5% NaCl solution at apotential of from about −0.7 to about −0.6 (volts versus SCE) than a7xxx alloy of similar size and shape, and which 7xxx alloy has a similarstrength and toughness to that of the new alloy.

Since the new alloy achieves better galvanic corrosion resistance and alower density than these 7xxx alloys, while achieving similar strengthand toughness, the new alloy is well suited as a replacement for these7xxx alloys. The new alloy may even be used in applications for whichthe 7xxx alloys would be rejected because of corrosion concerns.

With respect to fatigue, the alloys may realize a notched S/N fatiguelife of at least about 90,000 cycles, on average, for a 0.95 inch thickextrusion, at a max stress of 35 ksi. The alloys may achieve a notchedS/N fatigue life of at least about 75,000 cycles, on average for a 3.625inches thick extrusion at a max stress of 35 ksi. Similar values may beachieved for other wrought products.

Table 3, below, lists some extrusion properties of the new alloy andseveral prior art extrusion alloys.

TABLE 3 Properties of extruded alloys New Alloy 2099-T-83 2196-T85117150-T77 7055-T77 Thickness 0.500-2.000 0.500-3.000 0.236-0.9840.750-2.000 0.500-1.500 (inches) UTS (L) (ksi) 92 80 78.3 89 94 TYS (L)(ksi) 88 72 71.1 83 90 El. % (L) 7 7 5 8 9 CYS (ksi) 90 70 71.1 82 92Shear Ultimate 48 41 — 44 48 Strength (ksi) Bearing 110 104 99.3 118 128Ultimate Strength e/D = 1.5 (ksi) Bearing Yield 100 85 87 96 109Strength e/D = 1.5 (ksi) Bearing 150 135 136.3 152 167 Ultimate Strengthe/D = 2.0 (ksi) Bearing Yield 115 103 104.4 117 131 Strength e/D = 1.5(ksi) Tensile modulus 11.4 11.4 11.3 10.4 10.4 (E) - Typical (10³ ksi)Compressive 11.6 11.9 11.6 11.0 11.0 modulus (Ec) - Typical (10³ ksi)Density (lb./in³) 0.097 0.095 0.095 0.102 0.103 Specific TYS 9.07 7.587.48 8.14 8.74 (10⁵ in.) Toughness 27 — 24 27 (L-T) (ksi√in.) (typical)

As illustrated above, the new alloy realizes an improved combination ofmechanical properties relative to the prior art alloys. For example, andas illustrated in FIG. 2, the new alloy realizes an improved combinationof strength and modulus relative to the prior art alloys. As anotherexample, and as illustrated in FIG. 3, the new alloy realizes improvedspecific tensile yield strength relative to the prior art alloys.

Designers select aluminum alloys to produce a variety of structures toachieve specific design goals, such as light weight, good durability,low maintenance costs, and good corrosion resistance. The new aluminumalloy, due to its improved combination of properties, may be employed inmany structures including vehicles such as airplanes, bicycles,automobiles, trains, recreational equipment, and piping, to name a few.Examples of some typical uses of the new alloy in extruded form relativeto airplane construction include stringers (e.g., wing or fuselage),spars (integral or non-integral), ribs, integral panels, frames, keelbeams, floor beams, seat tracks, false rails, general floor structure,pylons and engine surrounds, to name a few.

The alloys may be produced by a series of conventional aluminum alloyprocessing steps, including casting, homogenization, solution heattreatment, quench, stretch and/or aging. In one approach, the alloy ismade into a product, such as an ingot derived product, suitable forextruding. For instance, large ingots can be semi-continuously casthaving the compositions described above. The ingot may then be preheatedto homogenize and solutionize its interior structure. A suitable preheattreatment step heats the ingot to a relatively high temperature, such asabout 955° F. In doing so, it is preferred to heat to a first lessertemperature level, such as heating above 900° F., for instance about925-940° F., and then hold the ingot at that temperature for severalhours (e.g., 7 or 8 hours). Next the ingot is heated to the finalholding temperature (e.g., 940-955° F. and held at that temperature forseveral hours (e.g., 2-4 hours).

The homogenization step is generally conducted at cumulative hold timesin the neighborhood of 4 to 20 hours, or more. The homogenizingtemperatures are generally the same as the final preheat temperature(e.g., 940-955° F.). Overall, the cumulative hold time at temperaturesabove 940° F. should be at least 4 hours, such as 8 to 20 or 24 hours,or more, depending on, for example, ingot size. Preheat andhomogenization aid in keeping the combined total volume percent ofinsoluble and soluble constituents low, although high temperatureswarrant caution to avoid partial melting. Such cautions can includecareful heat-ups, including slow or step-type heating, or both.

Next, the ingot may be scalped and/or machined to remove surfaceimperfections, as needed, or to provide a good extrusion surface,depending on the extrusion method. The ingot may then be cut intoindividual billets and reheated. The reheat temperatures are generallyin the range of 700-800° F. and the reheat period varies from a fewminutes to several hours, depending on the size of the billet and thecapability of the furnace used for processing.

Next, the ingot may be extruded via a heated setup, such as a die orother tooling set at elevated temperatures (e.g., 650-900° F.) and mayinclude a reduction in cross-sectional area (extrusion ratio) of about7:1 or more. The extrusion speed is generally in the range of 3-12 feetper minute, depending on the reheat and tooling and/or die temperatures.As a result the extruded aluminum alloy product may exit the tooling ata temperature in the range of, for example, 830-880° F.

Next, the extrusion may be solution heat treated (SHT) by heating atelevated temperature, generally 940-955° F. to take into solution all ornearly all of the alloying elements at the SHT temperature. Afterheating to the elevated temperature and holding for a time appropriatefor the extrusion section being processed in the furnace, the productmay be quenched by immersion or spraying, as is known in the art. Afterquenching, certain products may need to be cold worked, such as bystretching or compression, so as to relieve internal stresses orstraighten the product, and, in some cases, to further strengthen theproduct. For instance, an extrusion may have an accumulated stretch ofas little as 1% or 2%, and, in some instance, up to 2.5%, or 3%, or3.5%, or, in some cases, up to 4%, or a similar amount of accumulatedcold work. As used herein, accumulated cold work means cold workaccumulated in the product after solution heat treatment, whether bystretching or otherwise. A solution heat treated and quenched product,with or without cold working, is then in a precipitation-hardenablecondition, or ready for artificial aging, described below. As usedherein, “solution heat treat” includes quenching, unless indicatedotherwise. Other wrought product forms may be subject to other types ofcold deformation prior to aging. For example, plate products may bestretched 4-6% and optionally cold rolled 8-16% prior to stretching.

After solution heat treatment and cold work (if appropriate), theproduct may be artificially aged by heating to an appropriatetemperature to improve strength and/or other properties. In oneapproach, the thermal aging treatment includes two main aging steps. Itis generally known that ramping up to and/or down from a given or targettreatment temperature, in itself, can produce precipitation (aging)effects which can, and often need to be, taken into account byintegrating such ramping conditions and their precipitation hardeningeffects into the total aging treatments. In one embodiment, the firststage aging occurs in the temperature range of 200-275° F. and for aperiod of about 12-17 hours. In one embodiment, the second stage agingoccurs in the temperature range of 290-325° F., and for a period ofabout 16-22 hours.

The above procedures relates to methods of producing extrusions, butthose skilled in the art recognized that these procedures may besuitably modified, without undue experimentation, to produce sheet/plateand/or forgings of this alloy.

EXAMPLES Example 1

Two ingots, 23″ diameter×125″ long, are cast. The approximatecomposition of the ingots is provided in Table 4, below (all values inweight percent). The density of the alloy is 0.097 lb/in³.

TABLE 4 Composition of Cast Alloy Cu Li Zn Ag Mg Mn Balance 3.92% 1.18%0.52% 0.48% 0.34% 0.34% aluminum, grain structure control elements,incidental elements and impurities

The two ingots are stress relieved, cropped to 105″ lengths each andultrasonically inspected. The billets are homogenized as follows:

-   -   18 hour ramp to 930° F.;    -   8 hour hold at 930° F.;    -   16 hour ramp to 946° F.;    -   48 hour hold at 946° F.

(furnace requirements of −5° F., +10° F.)

The billets are then cut to the following lengths:

-   -   43″-qty of 1    -   31″-qty of 1    -   30″-qty of 1    -   44″-qty of 1

Final billet preparation (pealed to the desired diameter) for extrusiontrials are completed. The extrusion trial process involves evaluation of4 large press shapes and 3 small press shapes. Three of the large pressshapes are extruded to characterize the extrusion settings and materialproperties for an indirect extrusion process and one large press shapefor a direct extrusion process. Three of the four large press shapethicknesses extruded for this evaluation ranged from 0.472″ to 1.35″.The fourth large press shape is a 6.5″ diameter rod. The three smallpress shapes are extruded to characterize the extrusion settings andmaterial properties for the indirect extrusion process. The small pressshape thicknesses range from 0.040″ to 0.200″. The large press extrusionspeeds range from 4 to 11 feet per minute, and the small press extrusionspeeds range from 4 to 6 feet per minute.

Following the extrusion process, each parent shape is individually heattreated, quenched, and stretched. Heat treatment is accomplished atabout 945-955° F., with a one hour soak. A stretch of 2.5% is targeted.

Representative etch slices for each shape are examined and revealrecrystallization layers ranging from 0.001 to 0.010 inches. Some of thethinner small press shapes do, however, exhibit a mixed grain(recrystallized and unrecrystallized) microstructure.

Single step aging curves at 270 and 290° F. for large press shapes arecreated. The results indicate that the alloy has a high toughness, andat the same time approaching the static tensile strengths of acomparable 7xxx product (e.g., 7150-T77511).

To further improve the strength of the alloy, a multi-step age practiceis developed. Multi-step age combinations are evaluated to improve thestrength—toughness relationship, while also endeavoring to achieve thestatic property targets of known high strength 7xxx alloys. The finallydeveloped multi-step aging practice is a first aging step at 270° F. forabout 15 hours, and a second aging step at about 320° F. for about 18hours.

Corrosion testing is performed during temper development. Stresscorrosion cracking (SCC) tests are performed in accordance with ASTM G47and G49 on the sample alloy, and in the direction and stresscombinations of LT/55 ksi and ST/25 ksi. The alloys passes the SCC testseven after 155 days.

MASTMAASIS testing (intermittent salt spray test) is also performed, andreveals only a slight degree of exfoliation at the T/10 and T2 planesfor single and multi-step age practices. The MASTMAASIS results yield a“P” rating for alloys at both T/2 and T/10 planes.

The alloys are subjected to various mechanical tests at variousthicknesses. Those results are provided in Table 5, below.

TABLE 5 Properties of tested alloys (average) Thickness UTS TYS EL % CYSDensity Toughness Alloy Temper (inches) (L) (ksi) (L) (ksi) (L) (ksi)(lb./in³) (L-T) (ksi√in.) New T8  0.04-0.200 88.8 84.1 8.1 — 0.097 — NewT8 0.472 98.7 95.8 9.3 101 0.097 — New T8 0.787-1.35  94.6 90.8 9.4 93.60.097 27.6

As illustrated in Table 3, above, and via these results, the alloysrealize an improved combination of strength and toughness overconventionally extruded alloys 2099 and 2196. The alloys also realizesimilar strength and toughness relative to conventional 7xxx alloys 7055and 7150, but are much lighter, providing a higher specific strengththan the 7xxx alloys. The new alloys also achieve a much better tensileand compressive modulus relative to the 7xxx alloys. This combination ofproperties is unique and unexpected.

Example 2

Ten 23″ diameter ingots are cast. The approximate composition of theingots is provided in Table 6, below (all values are weight percent).The density of the alloy is 0.097 lb/in³.

TABLE 6 Composition of Cast Alloy Cast Cu Li Zn Ag Mg Mn Balance 1-A3.95% 1.18% 0.53% 0.50% 0.36% 0.26% aluminum, grain 1-B 3.81% 1.15%0.49% 0.49% 0.34% 0.28% structure control elements, incidental elementsand impurities

The ingots are stress relieved and three ingots of cast 1-A and threeingots of cast 1-B are homogenized as follows:

-   -   Furnace set at 940° F. and charge all 6 ingots into said        furnace;    -   8 hour soak at 925-940° F.;    -   Following 8 hour hold, reset the furnace to 948° F.;    -   After 4 hours, reset the furnace to 955° F.;    -   24 hour hold 940-955° F.

The billets are cut to length and pealed to the desired diameter. Thebillets are extruded into 7 large press shapes. The shape thicknessesrange from 0.75 inch to 7 inches thick. Extrusion speeds and pressthermal settings are in the range of 3-12 feet per minute, and at fromabout 690-710° F. to about 750-810° F. Following the extrusion process,each parent shape is individually solution heat treated, quenched andstretched. Solution heat treatments targeted 945-955° F., with soaktimes set, depending on extrusion thickness, in the range of 30 minutesto 75 minutes. A stretch of 3% is targeted.

Representative etch slices for each shape are examined and revealrecrystallization layers ranging from 0.001 to 0.010 inches. Multi-stepaging cycles are completed to increase the strength and toughnesscombination. In particular, a first step aging is at about 270° F. forabout 15 hours, and a second step aging is at about 320° F. for about 18hours.

Stress corrosion cracking tests are performed in accordance with ASTMG47 and G49 on the sample alloy, and in the direction and stresscombination of LT/55 ksi and ST/25 ksi, both located in the T/2 planes.The alloys pass the stress corrosion cracking tests.

MASTMAASIS testing (intermittent salt spray test) is also performed inaccordance with ASTM G85-Annex 2 and/or ASTM G34. The alloys achieve aMASTMAASIS rating of “P”.

Notched S/N fatigue testing is also performed in accordance with ASTME466 at the T/2 plane to obtain stress-life (S-N or S/N) fatigue curves.Stress-life fatigue tests characterize a material's resistance tofatigue initiation and small crack growth which comprises a majorportion of the total fatigue life. Hence, improvements in S-N fatigueproperties may enable a component to operate at a higher stress over itsdesign life or operate at the same stress with increased lifetime. Theformer can translate into significant weight savings by downsizing,while the latter can translate into fewer inspections and lower supportcosts.

The S-N fatigue results are provided in Table 7, below. The results areobtained for a net max stress concentration factor, Kt, of 3.0 usingnotched test coupons. The test coupons are fabricated as illustrated inFIG. 4. The test coupons are stressed axially at a stress ratio (minload/max load) of R=0.1. The test frequency is 25 Hz, and the tests areperformed in ambient laboratory air.

With respect to FIG. 4, to minimize residual stress, the notch should bemachined as follows: (i) feed tool at 0.0005″ per rev, until specimen is0.280″; (ii) pull tool out to break chip; (iii) feed tool at 0.0005″ perrev. to final notch diameter. Also, all specimens should be degreasedand ultrasonically cleaned, and hydraulic grips should be utilized.

In these tests, the new alloy showed significant improvements in fatiguelife with respect to the industry standard 7150-T77511 product. Forexample, at an applied net section stress of 35 ksi, the new alloyrealizes a lifetime (based on the log average of all specimens tested atthat stress) of 93,771 cycles compared to a typical 11,250 cycles forthe standard 7150-T77511 alloy. As a maximum net stress of 27.5 ksi, thealloy realizes an average lifetime of 3,844,742 cycles compared to atypical 45,500 cycles at net stress of 25 ksi for the 7150-T77511 alloy.Those skilled in the art appreciate that fatigue lifetime will dependnot only on stress concentration factor (Kt), but also on other factorsincluding but not limited to specimen type and dimensions, thickness,method of surface preparation, test frequency and test environment.Thus, while the observed fatigue improvements in the new alloycorresponded to the specific test coupon type and dimensions noted, itis expected that improvements will be observed in other types and sizesof fatigue specimens although the lifetimes and magnitude of theimprovement may differ.

TABLE 7 Notched S/N Fatigue Results Maximum net New alloy - 0.950 inchNew alloy - 3.625 inches stress (ksi) (cycles to failure) (cycles tofailure) 35 78,960 61,321 35 129,632 86,167 35 110,873 82,415 35 61,147— 35 105,514 — 35 76,501 — AVERAGE 93,711 76,634 27.5 696,793 27.52,120,044 27.5 8,717,390

The alloys are subjected to various mechanical tests at variousthicknesses. Those results are provided in Table 8, below.

TABLE 8 Properties of extruded alloys (averages) New Alloy New Alloy NewAlloy Thickness 0.750 0.850 3.625 (inches) UTS (L) (ksi) 93.5 100.1 92.6TYS (L) (ksi) 88.8 97.1 88.7 El. % (L) 10.4 9.9 7.9 CYS (ksi) 93.9 98.393.3 Shear Ultimate Strength (ksi) 52.1 51.6 53.1 Bearing Ultimate 112.8112.2 108.9 Strength e/D = 1.5 (ksi) Bearing Yield Strength 130.7 130.3124 e/D = 1.5 (ksi) Bearing Ultimate Strength 132.2 132.5 127.1 e/D =2.0 (ksi) Bearing Yield Strength 168.4 168.1 160.9 e/D = 1.5 (ksi)Tensile modulus (E) - Typical 11.4 11.4 11.4 (10³ ksi) Compressivemodulus (Ec) - 11.6 11.7 11.7 Typical (10³ ksi) Density (lb./in³) 0.0970.097 0.097 Specific Tensile Yield 9.15 10.0 9.14 Strength (10⁵ in.)Toughness — 31.8 23.3 (L-T) (ksi√in.)

Galvanic corrosion tests are conducted in quiescent 3.5% NaCl solution.FIG. 5 is a graph illustrating the galvanic corrosion resistance of thenew alloy. As illustrated, the new alloy realizes at least a 50% lowercurrent density than alloy 7150, the degree of improvement varyingsomewhat with potential. Notably, at a potential of about −0.7V vs. SCE,the new alloy realizes a current density that is over 99% lower thanalloy 7150, the new alloy having a current density of about 11 uA/cm²,and alloy 7150 having a current density of about 1220 uA/cm²((1220−11)/1220=99.1% lower).

While various embodiments of the present alloy have been described indetail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present disclosure.

What is claimed is:
 1. An aluminum alloy consisting of: 3.6-4.0 wt. %Cu; 1.1-1.2 wt. % Li; 0.4-0.55 wt. % Ag; 0.25-0.45 wt. % Mg; 0.4-0.6 wt.% Zn; 0.25-0.45 wt. % Mn; and 0.05-0.15 wt. % Zr; the balance beingaluminum and incidental elements and impurities.
 2. The aluminum alloyof claim 1, wherein the alloy contains at least 3.7 wt. % Cu.
 3. Thealuminum alloy of claim 1, wherein the alloy contains at least 3.8 wt. %Cu.
 4. The aluminum alloy of claim 1, wherein the alloy contains notgreater than 3.95 wt. % Cu.
 5. The aluminum alloy of claim 1, whereinthe alloy contains at least 0.45 wt. % Zn.
 6. The aluminum alloy ofclaim 1, wherein the alloy contains not greater than 0.55 wt. % Zn. 7.The aluminum alloy of claim 1, wherein the alloy contains at least 0.45wt. % Ag.